ACQUISITION - 2017 Acquisition (Details)					9 Months Ended
	Nov. 06, 2017 USD ($)	Nov. 01, 2017 USD ($)	Aug. 01, 2017 USD ($) item	Jun. 01, 2017 USD ($)	Sep. 30, 2017 USD ($)
ACQUISITION
Return on investment distribution classified as investing activities					$ 3,000,000
Iroquois
ACQUISITION
Interest acquired (as a percent)			49.34%	49.34%	49.34%
Option to acquire (as a percent)				0.66
Amount of final purchase price adjustments				$ 19,000,000
Additional consideration on surplus cash				28,000,000
Net purchase price				710,000,000
Outstanding debt				164,000,000
Payment for option to acquire				1,000
Iroquois net purchase price
Net Purchase Price				593,000,000
Less: TransCanada's carrying value of Iroquois at June 1, 2017				223,000,000
Excess purchase price				370,000,000
Purchase price before assumption of debt				710,000,000
Reduction in partner's equity under equity method investments				$ 370,000,000
Iroquois | Cash Distribution Paid
ACQUISITION
Return on investment distribution classified as investing activities			$ 2,600,000
Iroquois | Cash Distribution Paid | Subsequent Events
ACQUISITION
Return on investment distribution classified as investing activities	$ 5,200,000	$ 2,600,000
Iroquois | Investing Activities
ACQUISITION
Return on investment distribution classified as investing activities			$ 28,000,000
Number of quarters for distribution of surplus cash | item			11
Iroquois | TransCanada
ACQUISITION
Additional consideration on surplus cash			$ 28,000,000
Portland Natural Gas Transmission System
ACQUISITION
Interest acquired (as a percent)				11.81%
Interest acquired by Partnership (as a percent)				61.71%
Amount of final purchase price adjustments				$ 3,000,000
Net purchase price				55,000,000
Outstanding debt				5,000,000
PNGTS purchase price
Current assets				25,000,000
Property, plant and equipment, net				294,000,000
Current liabilities				(4,000,000)
Deferred state income taxes				(10,000,000)
Long-term debt, including current portion				(41,000,000)
Net Purchase Price				264,000,000
Non-controlling interest				(100,000,000)
Carrying value of pre-existing Investment in PNGTS				(132,000,000)
TransCanada's carrying value of the acquired 11.81 percent interest at June 1, 2017				32,000,000
Excess purchase price over net assets acquired				21,000,000
Total cash consideration				53,000,000
Purchase price before assumption of debt				55,000,000
Final working capital adjustment				3,000,000
Reduction in partner's equity due to excess purchase price				21,000,000
Portland Natural Gas Transmission System | Iroquois
ACQUISITION
Purchase price				765,000,000
Amount of final purchase price adjustments				$ 50,000,000